The bottom line: Smokers who used e-cigarettes quit smoking at half the rate of smokers who did not use e-cigarettes (9.4% among e-cig users vs. 18.9% among those who didn’t). The kind of product and intensity of use did not affect the results, nor did the presence of flavors.

Here is the abstract:

BACKGROUND: The potential of electronic nicotine delivery systems (ENDS) to reduce the cardiovascular and other disease risks of smoking is of great interest. While many smokers report using ENDS for cessation, their impact under real-world use patterns and conditions on adult smokers' quitting behavior is uncertain. The objective of this study was to generate more recent and comprehensive evidence on the effect of "real world" ENDS use on the population quit rates of adult smokers while taking account of frequency and duration of use, device type, e-liquid flavor, and reasons for use.

METHODS AND FINDINGS: We conducted a population-based, prospective cohort study of a random probability sample of 1284 U.S. adult smokers recruited in August/September 2015 and re-contacted one-year later (September 2016) from GfK's KnowledgePanel, a national, probability-based web-panel designed to be representative of non-institutionalized U.S. adults. Among the 1081 baseline smokers who remained members of KnowledgePanel, 858 completed the follow-up survey. The primary outcome was smoking abstinence for at least 30 days prior to follow-up. Secondary outcomes were making a quit attempt during the 12-month study period and number of cigarettes smoked per day at follow-up. The adjusted odds of quitting smoking were lower for those that used ENDS at baseline (9.4%, 95% CI = 5.22%-16.38%; AOR = 0.30, 95% CI = 0.13-0.72) compared to smokers who did not use at ENDS (18.9%, 95% CI = 14.24%-24.68%). Smokers who used ENDS daily at some point during the study period were also less likely to quit smoking than nonusers (AOR = 0.17; 95% CI = 0.04-0.82). Limited ability to draw causal inferences from the observational design and a lack of biochemical verification of quitting smoking or ENDS use are limitations of this study.

CONCLUSIONS: We found no evidence that ENDS use, within context of the 2015-2016 US regulatory and tobacco/vaping market landscape, helped adult smokers quit at rates higher than smokers who did not use these products. Absent any meaningful changes, ENDS use among adult smokers is unlikely to be a sufficient solution to obtaining a meaningful increase in population quit rates. Additional research is needed to reconcile the divergent literature and monitor the impact of ENDS in an environment of rapidly evolving markets and regulatory policies.

It is particularly noteworthy that the GSU group has been relatively sympathetic to the idea of e-cigarettes for smoking cessation and harm reduction (which is reflected in some of the language in the paper). For example, rather than their statement that, “[w]e found no evidence that ENDS use, within context of the 2015-2016 US regulatory and tobacco/vaping market landscape, helped adult smokers quit at rates higher than smokers who did not use these products,” I would have said “we find statistically significant evidence for depressed quitting.”

But that is quibbling. But they are clear about the overall negative (in the sense of showing less quitting cigarettes among e-cigarette users) results that they found.

I added this paper to my running meta-analysis of the relationship between e-cigarette use and smoking cessation that Sara Kalkhoran and I published in 2016. That analysis of 21 studies found an adjusted OR for quitting associated with e-cigarettes of 0.72 (95% CI 0.57-0.91). Adding this latest study brings the number of studies to 37 and yields an adjusted odds ratio of quitting of 0.75 (95% CI 0.59-0.95). The stability of this estimate at the number of studies and their general quality has increased is additional evidence that the conclusion that e-cigs depress smoking cessation is real.